In the process of resistance welding of workpieces, the members to be joined are heated by the passage of a strong electrical current in the area of the intended joint and are pressed together in that area while soft and plastic. Almost all metallic objects can be welded together in this manner, provided only tht they exhibit a sufficiently high degree of electrical conductivity. However, the welding of objects made from non-ferrous metals presents special problems due to their often very high electrical conductivity which requires the use of very heavy currents to generate the necessary heat. The high thermal conductivity of these metals places limits on the duration of current flow so as to prevent an undesired spreading of the heat to other parts of the workpiece. These problems occur especially when thin copper wires are to be welded to relatively massive steel workpieces. The different melting points of copper and steel make it necessary that the heat of welding be kept within relatively narrow limits so as to ensure adequate commingling of metal by welding while preventing excessive deformation of the copper wire in the area of the weld. Various surface treatments have been proposed but these all strongly affect the welding process and thus require adjustment of the welding variables, i.e. the pressure, the current and the duration. It has been proposed to improve the weldability of copper wires by coating them with a layer of tin or cadmium in order to increase the transition resistance at the welding point but it has been found that these coatings require reduced welding currents and, in some cases, special welding electrode materials. More recently, copper wires have been coated with a surface layer of nickel. Such a coating further reduces the weldability of the wires due to the substantially higher melting point of nickel as compared with that of copper. Thus, it is known from British Pat. No. 1,018,873 to coat copper wires and plates with a layer of nickel prior to joining by resistance welding. It is the purpose of the nickel layer to prevent melting of the copper in the area of contact of the electrodes and thus to prevent adhesion to the welding electrodes.